We cloned a novel member of the ETS family of transcription factors called MEF, from a human megakaryocytic leukemia cell line. MEF is a strong transcriptional activator that is expressed in immature and mature hematopoietic cells. We have demonstrated that MEF is phosphorylated by the cyclin A/CDK2 complex and have preliminarily shown that sequential phosphorylation leads to its degradation by the ubiquitin proteasome degradation pathway. After generating murine cDNA and genomic MEF clones, we created MEF "knock-out" mice which have defects in NK and NK T cell development and abnormalities in the hematopoietic stem cell compartment. MEF -7- NK cells cannot lyse tumor cells, probably because perforin, an MEF target gene, is not expressed. Based on the phenotype of these mice, and the cell cycle dependent regulation of MEF function, we propose to further characterize the biological effects of MEF on hematopoietic stem cells, NK and NK T cells by utilizing MEF-null mice for several bone marrow transplant models (and for target gene identification) and by overexpressing MEF in hematopoietic progenitor cells. We will define the mechanisms of MEF action by identifying and characterizing MEF target genes in hematopoietic cells and by further characterizing the cell cycle dependent regulation of MEF expression. The studies proposed will define the role of MEF in regulating gene expression involved in hematopoietic stem cell behavior and innate immunity. These studies will contribute greatly to advances in the fields of stem cell transplantation and immune regulation.